This invention relates to branched thermoplastic copolyesters and more particularly to copolyesters wherein the branching is effected by the presence of a branching agent during the preparation of said copolyester. The branching agent serves to lessen substantially the time needed to produce a copolyester of acceptable melt viscosity; that is to say, the presence of branching within the copolyester serves to lessen the reaction time which is necessary to produce a copolyester of desired viscosity. The branching agent must be present in a critical amount which will subsequently be defined.
Linear thermoplastic copolyesters have been suggested heretofore for various purposes, particularly for the production of films and fibers. Recently, certain polymers of this type have been found suitable for particular application such as, for example, molded vehicle tires where superior tear strength, tensile strength, flex life and abrasion resistance are necessary. The copolyesters, which have these desirable features, require a rather extended period of reaction time to reach a suitable melt index, for extrusion, injection molding and blow molding applications e.g. about 10-1 at 220.degree. C. Consequently, a method for producing the copolyester which will minimize the time required to reach the desired viscosity is needed in order to minimize the investment in polymerization equipment or conversely to increase the output of a given set of equipment.
According to this invention there is provided a method for producing such a branched thermoplastic copolyester, which minimizes the time required for producing the copolyester having a desired melt index. (e.g. 10 to 1). The copolyester possesses all of its original desired properties such as superior tear strength, tensile strength, flex life etc. and has improved high temperature properties relative to the unbranched copolyester. In addition, the good processibility of the copolyester is retained and can even be improved for use in extrusion and blow molding. For certain copolyesters the present method permits the preparation of polymers having superior properties relative to the corresponding unbranched polymer.
The above described improvement is effected by preparing the copolyester in the presence of a critical amount of a branching agent, said branching agent having the general formula (HO).sub.a X (COOH).sub.b, wherein X is a polyfunctional radical, a = 0-6, b = 0-4 and a + b = 3-6.
The copolyester which is prepared in the presence of the branching agent of the instant invention is a thermoplastic copolyester consisting essentially of a multiplicity of recurring intralinear long chain and short chain ester units connected head-to-tail through ester linkages, said long chain ester units being represented by the following structure: ##EQU1## AND SAID SHORT CHAIN ESTER UNITS BEING REPRESENTED BY THE FOLLOWING STRUCTURE: ##EQU2## wherein: G is a divalent radical remaining after removal of terminal hydroxyl groups from poly(alkylene oxide) glycols having a carbon-to-oxygen ratio of about 2.0-4.3 and a molecular weight between about 400 and 4,000;
R is a divalent radical remaining after removal of carboxyl groups from a dicarboxylic acid having a molecular weight less than about 300; and PA1 D is a divalent radical remaining after removal of hydroxyl groups from a low molecular weight diol having a molecular weight less than about 250.
It is preferred that the short chain ester units constitute about 15-95% by weight of the copolyester and at least about 50% of the short chain ester units be identical.
The segmented thermoplastic copolyetherester described above is prepared by reacting at elevated temperature and by conventional means: a dicarboxylic acid having a weight less than about 300 or its corresponding ester equivalent, which would be represented by the formula ##EQU3## wherein R is a divalent radical and R' is independently selected from the group consisting of hydrogen and C.sub.1 -C.sub.10 alkyl; a poly(alkylene oxide) glycol having a molecular weight of about 400-4000 and a carbon to oxygen ratio of about 2.0-4.3 which would be represented by the formula HO--G--OH wherein G is a divalent radical and a diol having a molecular weight less than about 250 which would be represented by the formula HO--D--OH wherein D is a divalent radical.
The reaction between the dicarboxylic acid, the poly(alkylene oxide) glycol and the diol is conducted in the presence of a branching agent. The branching agent has the general formula (HO).sub.a X (COOH).sub.b and has a molecular weight of about 92 to 5000. X is a polyfunctional radical, a = 0 to 6, b = 0-4 and the sum of a + b = 3 to 6. It is critical to the instant invention that the branching agent be present in the amount of about 0.3 to 1.2 equivalents per 100 moles of dicarboxylic acid. When the branching agent is present in amounts of less than 0.3 equivalent, the increase in output of copolyester having the desired melt viscosity is insignificant. At concentrations above 1.2 equivalents certain properties such as tensile strength and flex resistance of the finished copolyester are affected adversely.
In an embodiment of the instant invention the diol and the dicarboxylic acid may be reacted independently initially and then the product of the reaction reacted in turn with the glycol. The branching agent may be added to either stage when this procedure is used.